Cold cathode for electron discharge devices



July 12, 1960 13. G. FIRTH cow CATHODEFOR ELECTRON DISCHARGE DEVICES Filed April 24, 1959 INVENTOR BERNARD G. HRTH BY 9 M 1. Jim:

ATTO R N EYS 2,945,151 Patented July 12,1960

COLD CATHODE FOR ELECTRON DISCHARGE DEVICES Bernard G. Firth, Newark, N.J., assignor to Tang-Sol Electric Inc., a corporation of Delaware Filed Apr. 24, 1959, Ser. No. 808,618

'8 Claims. (Cl. '313-346) This invention relates to cathode coatings for use in electron discharge devices. It has particular reference to coatings for unheated cathodes used in vacuum tubes wherein the transfer of current is due entirely'to the flow of electrons.

Hard vacuum tubes with cold cathodes have been designed and made for some time. The coating generally used is a form of magnesium oxide deposited on a metal base in a thin and porous layer. Electron emission is started by the application of an auxiliary stream of ionizing particles and the emission is maintained by an electric field set up between the metal cathode base and a sustaining electrode. The sustaining electrode is generally a grid-like arrangement mounted in parallel relationship to the metal base and connected to the metal base in series with a source of potential and a ballast resistor.

Cold cathodes made of pure magnesium oxide give good results in vacuum tubes and such tubes, when including control grids and an anode, may be used as amplifiers, oscillators, modulators, and gates. However, such cathodes are relatively difiicult to start and develop during emission at high voltage drop between the metal base and the outer cathode surface.

An effort has been made to increase the facility for starting, reduce the cathode voltage drop, and increase the electron emission. I have found that a small amount of sodium when intimately mixed with the magnesium in the form of sodium carbonate results in a cathode coating having a cathode voltage drop reduced by about an electron emission increased by at least 10% and having a starting time reduced by a factor of five.

One of the objects of this invention is to provide an improved cold cathode coating for electron discharge devices which avoids one or more of the disadvantages and limitations of prior art coatings.

Another object of the invention is to increase the emission of electrons from a cold cathode coating under the action of an electrostatic field.

Another object of the invention is to reduce the voltage of the field necessary to maintain emission.

Another object of the invention is to increase the facility for starting the electron emission after the electric field has been applied.

The invention comprises a cold cathode for an electron discharge device and includes a conducting base and a coating on the base composed of magnesium oxide and sodium oxide. The sodium oxide is evenly distributed within the magnesium oxide and is from 0.1 to 2.0% of the magnesium oxide.

For a better understanding of the present invention, reference is made to the following description taken in connection with the accompanying drawings.

Fig. 1 is a cross sectional view of an electron discharge device having a cold cathode.

Fig. 2 is a partial cross sectional view of the device shown in Fig. 1 taken along line 2-2 of that figure.

Referring now to the drawings, the discharge components are housed within an envelope 10 which may be glass. Lead-in conductors 11 are positioned in the envelope at one end of the device for connection to an external circuit. The discharge components include an anode 12, a control grid 13, a sustaining grid 14, and a hollow cathode structure 15.

The cathode includes a metal base covered with porous magnesium oxide activated with sodium oxide. The cathode base is generally made hollow so that a heater 16'may be positioned within the hollow space and aid in the processing of the coating when it is first formed. The heater is not used during the life of the tube as an electron discharge device.

In order to start the electron emission from the cathode, an electric field'is first applied between thesustaining grid 14 and the cathode base 15 and then a stream of electrons is sent through the discharge space from an out-.

Magnesium oxide, MgO -grams 20 Magnesium carbonate, MgCO do 20 Sodium carbonate, NaCO a ..milligrams Amyl acetate cc 300 The batch is ground in a ball mill and then, after the addition of 300 co. more amyl acetate, is sprayed onto the base metal, preferably nickel, of the cathode structure 15.

After the sprayed cathodes are dry, they are each mounted and sealed in a vacuum tube envelope, complete with anode 12 and the control electrodes 13 and 14 to be used in the operation of the tube as an electron dischargedevice. The tubes are now processed in a manner similar to hot cathode tubes to remove the air and moisture from the tube components. This process, which includes heating the cathodes by means of the cathode heater 16 in low pressure oxygen, changes the carbonates to oxides and leaves the coating in a porous state.

The anode 12 and cathode 15 are now connected to a source of potential in series with a ballast resistor and the discharge device is run on low power anode current for one or two hours to age the device and to diffuse the sodium oxide throughout the cathode coating.

The amount of sodium must be kept within a restricted range. Too little sodium does not provide the added electron emission while too much sodium in the cathode coating has a tendency to evaporate to the sustaining grid 14 and cause grid emission with a resultant grid current and reduced efiiciency. Experiment has shown that sodium added to the batch in the ratio of from one milligram to twenty milligrams of sodium carbonate for each gram of magnesium salt gives beneficial results.

Cold cathodes formed in the above described manner are easy to start, have a reduced cathode voltage drop, and are capable of electron emission of the order of 100 milliamperes per square centimeter with proper cooling facilities.

The foregoing disclosure is merely illustrative of the principle of this invention and is not to be interpreted in a limiting sense. The only limitations are to be determined from the scope of the appended claims.

What is claimed is:

1. A cold cathode comprising: a metal base and a coating thereon, said coating including magnesium oxide and sodium oxide, the amount of magnesium oxide being at least 50 times greater in weight than the amount of sodium oxide.

2. A cold cathode for an electron discharge device comprising: a conducting base and a coating thereon, said coating including a porous layer of magnesium oxide and sodium oxide, said sodium oxide evenly distributed within said magnesium oxide, the amount of sodium oxide being within the range of 0.1 to 2.0% by weight of said magnesium oxide. i

3. A cold cathode as set forth inclaim 2 wherein said metal base is nickel.

4. A cold cathode as set forth in claim 2 wherein said metal base is tubular and encloses a heater element used during the formation of the cathode coating.

5. An electron discharge device comprising: an anode, at least one control electrode, and a cold cathode, said cold cathode including a metal base and a coating of magnesium oxide and sodium oxide, theamount of sodium oxide being within the range of 0.1 to 2.0% by weight of said magnesium oxide. i

6. An electron discharge device comprising: an anode, a sustaining electrode, and a cold cathode, said cold cathode including a metal base and a coating of magnesium oxide and sodium oxide, the amount of sodium 4 oxide being within the range of 0.1 to 2.0% by weight of magnesium oxide, said sustaining electrode formed of a plurality of spaced conductive wires in a plane parallel to the surface of the cold cathode.

7. An electron discharge device comprising: an anode, an emission sustaining electrode, and a cold cathode, said cold cathode including a metal base and a coating of magnesium oxide and sodium oxide, the amount of sodium oxide being within the range of 0.1 to 2.0% by weight of said magnesium oxide.

8. An electron discharge device comprising: an anode, a sustaining electrode, at least one control electrode, and a cold cathode, said cold cathode including a metal base and a coating of magnesium oxide and sodium oxide, the amount of sodium oxide being within the range of 0.1 to 2.0% by weight of magnesium oxide, said sustaining electrode formed of a plurality of spaced conductive wires in a plane parallel to the surface of the cold cathode.

No references cited, 

1. A COLD CATHODE COMPRISING: A METAL BASE AND A COATING THEREON, SAID COATING INCLUDING MAGNESIUM OXIDE AND SODIUM OXIDE, THE AMOUNT OF MAGNESIUM OXIDE BEING AT LEAST 50 TIMES GREATER IN WEIGHT THAN THE AMOUNT OF SODIUM OXIDE. 